Current and future generations of metal-oxide-semiconductor field-effect transistors (MOSFETs) require tight control of parasitic capacitance while simultaneously optimizing metal-semiconductor contact resistance. In FinFET structures, maximizing the contact area can be achieved by creating a contact that wraps around the fin or by growing faceted epitaxial contacts, and then wrapping metal around the faceted epitaxial contact. For both MOSFET and FinFET processing, extensive etching and cleaning is required and generally these processes increase the critical dimension (CD) of the recessed feature or contact (via) hole. For back end of line (BEOL) optimization, it is likewise important to etch/clean through via holes without degrading via hole CD.
Reactive ion etching (RIE) is commonly used for etching recessed features in substrates and films. However, it is known that the RIE leaves fluorocarbon residues on the exposed surface after etching of silicon dioxide layers. These residues can be nonvolatile, chemically and thermally stable, and if not removed, they can result in high contact resistance and degradation of interfacial quality. Common residue removal processes include plasma cleaning in combination with a wet cleaning or a wet step alone. Fluorocarbon residues can, for example, be removed using an oxygen plasma that oxidizes the residues to form oxides (e.g., SiOx). Thereafter, the oxides may be removed using an aqueous HF solution. Although this type of residue removal is very effective, Si or SiOx consumption from the sidewall of the recessed features can result in an increase in the CD of the recessed features.